1. Field of the Disclosure
The present disclosure relates to a channel sensing method, and more particularly, to a channel access method including a channel sensing operation and a channel occupation operation that are performed by a transmitting node or a receiving node for uplink signal transmission in a wireless communication system using an unlicensed band.
2. Description of the Related Art
To meet a demand for radio data traffic that is on an increasing trend since commercialization of a 4G communication system, efforts to develop an improved 5G communication system or a pre-5G communication system have been conducted. For this reason, 5G communication systems or the pre-5G communication systems can be called a beyond 4G network communication system or a post LTE system.
To achieve a high data transmission rate, the 5G communication system is considered to be implemented in a super high frequency (mmWave) band (e.g., a 60 GHz band). To relieve a path loss of a radio wave and increase a transfer distance of a radio wave in the super high frequency band, in the 5G communication system, technologies such as beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MIMO), an array antenna, analog beam-forming, and a large scale antenna have been used. Further, to improve a network of the system, in the 5G communication system, technologies such as an evolved small cell, an advanced small cell, a cloud radio access network (RAN), an ultra-dense network, a device to device communication (D2D), a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMP), and interference cancellation have been developed. In addition, in the 5G system, hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC), which are an advanced coding modulation (ACM) scheme and a filter bank multi carrier (FBMC), a non orthogonal multiple access (NOMA), and a sparse code multiple access (SCMA) that are an advanced access technology, and so on have been developed.
Meanwhile, the Internet has evolved from in a human-centered connection network through which a human being generates and consumes information to an internet of things (IoT) network that transmits and receives information between distributed components such as things and processes the information. The internet of everything (IoE) technology in which the big data processing technology is combined with the IoT technology by connection with a cloud server has also emerged. To implement the IoT, technology elements, such as a sensing technology, wired and wireless communication and network infrastructure, a service interface technology, and a security technology, have been required. Recently, technologies such as a sensor network, machine to machine (M2M), and machine type communication (MTC) for connecting between things has been researched. In the IoT environment, an intelligent internet technology (IT) service that creates a new value in human life by collecting and analyzing data generated in the connected things may be provided. The IoT may be applied to fields, such as a smart home, a smart building, a smart city, a smart car or a connected car, a smart grid, health care, smart appliances, and an advanced healthcare service, by fusing and combining the existing information technology with various industries.
Therefore, various attempts to apply the 5G communication system to the IoT network have been conducted. For example, the 5G communication technologies, such as the sensor network, the M2M, and the MTC, have been implemented by techniques such as beamforming, MIMO, and the array antenna. An example of the application of the cloud radio access network (cloud RAN) as the big data processing technology described above may also be the fusing of 5G technology with the IoT technology.
To meet a demand for wireless data traffic, discussions are underway to develop communication methods in various fields. For example, there are user equipment (UE) to UE communication, a frequency integration system for operating a plurality of cells, and a multi-antenna system using a large-scale antenna, or the like.
In recent years, a wireless communication system has been developed as a high-speed and high-quality wireless packet data communication system to provide a data service and a multimedia service in addition to provision of early voice-oriented service. In order to support the high-speed and high quality wireless packet data transmission service, various wireless communication standards such as high speed downlink packet access (HSDPA), high speed uplink packet access (HSUPA), long term evolution (LTE), long term evolution advanced (LTE-A) of 3rd generation partnership project (3GPP), high rate packet data (HRPD) of 3GPP2, and 802.16 of Institute of Electrical and Electronics Engineers (IEEE) have been developed. In particular, the LTE/LTE-A (LTE) has been continuously developed and progressed to improve system throughput and frequency efficiency. Typically, in the case of the LTE system, data transmission rate and system throughput may be significantly increased by using a frequency integration technology (carrier aggregation, (CA)) capable of operating the system using a plurality of frequency spectra. However, the frequency spectrum in which the LTE system currently operates is a licensed band or licensed carrier that an operator may use with its own authority. When a frequency spectrum (e.g., 5 GHz or less) in which the wireless communication service is typically provided, since it is already occupied and used by other operators or other communication systems, it may be difficult for the operator to secure a plurality of licensed band frequencies. Therefore, it is difficult to increase the system throughput using the CA technology. Accordingly, in order to process increasing mobile data in a situation in which it is hard to secure the licensed band frequency as described above, recently, a technology for utilizing the LTE system in an unlicensed band or unlicensed carrier has been researched (e.g., LTE in unlicensed (LTE-U) and licensed-assisted access (LAA)). Among the unlicensed spectra, particularly, 5 GHz spectrum is used by the relatively smaller number of communication devices as compared to 2.4 GHz unlicensed band, and may utilize significantly wide bandwidth, thus it is relatively easy to secure additional frequency spectrum. In other words, the licensed band frequency and the unlicensed band frequency may be utilized by using the LTE technology aggregating and using a plurality of frequency spectra, that is, the CA technology. In other words, an LTE cell in a licensed band may be set as PCell (or Pcell), an LTE in an unlicensed band (LAA cell or LTE-U cell) may be set as SCell (or Scell), such that the LTE system may be operated in the licensed band and the unlicensed band using the existing CA technology. The system may also be applied to a dual-connectivity environment, in which the licensed band and the unlicensed band are connected by a non-ideal backhaul, as well as the CA, in which the licensed band and the unlicensed band are connected by an ideal backhaul. However, in the present disclosure, the description will be made under the assumption of the CA environment in which the licensed band and the unlicensed band are connected by an ideal backhaul.
Generally, the LTE/LTE-A system is a method of transmitting data by using an orthogonal frequency division multiple access (OFDM) transmission scheme. In the OFDM scheme, a modulation signal is positioned at a two-dimensional resource configured of a time and a frequency. Resources on a time-axis are distinguished from each other by different OFDM symbols, and are orthogonal to each other. Resources on a frequency-axis are distinguished from each other by different sub-carriers, and are also orthogonal to each other. That is, in the OFDM scheme, when a specific OFDM symbol on the time-axis is designated and a specific sub-carrier on the frequency-axis is designated, one minimum unit resource may be indicated. The indicated minimum unit resource can be referred to as a resource element (RE). Different REs are orthogonal to each other even after passing through a frequency selective channel, therefore signals transmitted through different REs may be received by a reception side without causing interference with each other. In the OFDM communication system, a downlink bandwidth includes a plurality of resource blocks (RB), and each physical resource block (PRB) may include 12 sub-carriers arranged along the frequency-axis and 14 or 12 OFDM symbols arranged along the time-axis. Here, the PRB can be a basic unit for resource allocation.
A reference signal (RS) that is received from a base station, is a signal allowing a terminal to estimate a channel. In an LTE communication system, a demodulation reference signal (DMRS) can be included as one of a common reference signal (CRS) and an exclusive reference signal. A CRS that is a reference signal transmitted across entire downlink bandwidth, may be received by all terminals, and can be used in channel estimation, feedback information configuration of the terminal, or demodulation of a control channel and a data channel. A DMRS that is also a reference signal transmitted across entire downlink bandwidth, can be used in data channel modulation and channel estimation of a specific terminal, but is not used in feedback information configuration, unlike the CRS. Therefore, The DMRS can be transmitted through the PRB resource to be scheduled by the terminal.
On the time-axis, a subframe includes two slots, a first slot and a second slot, each having a length of 0.5 msec. A physical dedicated control channel (PDCCH) region that is a control channel region and an enhanced PDCCH (ePDCCH) region that is a data channel region are divided on the time-axis and then transmitted. This is to rapidly receive and demodulate the control channel signal. In addition, the PDCCH region is positioned across the entire downlink spectrum and has a form in which one control channel is divided into small units of control channels to be dispersed in the entire downlink spectrum. An uplink is largely divided into a control channel (PUCCH) and a data channel (PUSCH), and a response channel for the downlink data channel and other feedback information are transmitted through the control channel when the data channel is not present, and are transmitted through the data channel when the data channel is present.
FIGS. 1A and 1B are diagrams illustrating a conventional communication system to which the present disclosure can be applied.
Referring to FIGS. 1A and 1B, FIG. 1A illustrates a case in which an LTE cell 102 and an LAA cell 103 coexist in one small base station 101 in a network, and a terminal 104 performs transmission and reception of data with the base station 101 through the LTE cell 102 and the LAA cell 103. Schemes other than a duplex scheme of the LTE cell 102 or the LAA cell 103 can also be used. A cell performing data transmission and data reception by using a licensed band may be assumed as the LTE cell 102 or PCell, and a cell performing data transmission and data reception by using an unlicensed band may be assumed as the LAA cell 103 or SCell. However, when the LTE cell is PCell, uplink transmission may be performed only through the LTE cell 102.
FIG. 1B illustrates a case in which an LTE macro base station 111 for achieving wide coverage in the network and an LAA small base station 112 for increasing data transmission amount are installed; schemes other than a duplex scheme of the LTE macro base station 111 or the LAA small base station may also be used. In this case, the LTE macro base station 111 may also be replaced by an LTE small base station. Further, when the LTE base station is PCell, uplink transmission may be performed only through the LTE base station 111. The LTE base station 111 and the LAA base station 112 can be assumed to have an ideal backhaul network. Accordingly, fast X2 communication (or interface) 113 between the base stations is possible, such that even when the uplink transmission is performed only through the LTE base station 111, the LAA base station 112 may receive relevant control information from the LTE base station 111 through the X2 communication 113 in real time. Methods in accordance with the present disclosure may be applied to both of the systems in FIG. 1A and FIG. 1B.
In general, the unlicensed band is used in a manner that a plurality of devices share the same frequency spectrum or channel. The devices using the unlicensed band may be systems different from each other. Therefore, general operation of the devices operated in the unlicensed band for coexistence of various devices is as follows.
A transmitting device requiring transmission of a signal including a data signal, a control signal, or the like, confirms, with respect to the unlicensed band or a channel in which the signal transmission is performed, channel occupancy state of other devices before performing the signal transmission, and may occupy the channel depending on the determined channel occupancy state. The operation as described above is generally called listen-before-talk (LBT). In other words, the transmitting device needs to determine whether the transmitting device may occupy the channel according to a predefined or preset method. A method for sensing the channel may be defined or set in advance. Further, the time for sensing the channel may be defined or set in advance or randomly selected within a specific range. Moreover, the channel sensing time may be set in proportion to a set maximum channel occupancy time. A channel sensing operation for determining whether the channel may be occupied as described above may be set to be different depending on an unlicensed frequency spectrum in which the operation is performed, or regional and national regulation. For example, currently, in the United States, the unlicensed band may be used without a separate channel sensing operation except an operation for radar sensing in a frequency spectrum of 5 GHz.
The transmitting devices to use the unlicensed band may sense whether other devices use the corresponding channel through the foregoing channel sensing operation (or LBT) as described above, and use the channel by occupying the channel when it is sensed that the channel is not occupied by other devices in the channel. The devices using the unlicensed band may operate by defining or setting a maximum channel occupancy time for which the devices may continuously occupy a channel after the channel sensing operation, in advance. The maximum channel occupancy time may be defined in advance according to regulation defined in accordance with a frequency spectrum, a region, or the like, or may be separately set by a base station in a case of other devices, e.g., a terminal. The channel occupancy time may be set to be different depending on an unlicensed band or regional and national regulations. For example, in Japan, the maximum channel occupancy time is set to 4 ms in the unlicensed band of 5 GHz. Meanwhile, in Europe, the continuous channel occupancy time may be set to 10 ms or 13 ms. The devices occupying the channel for the maximum channel occupancy time may perform the channel sensing operation again, and then reoccupy the channel according to the channel sensing result.
The channel sensing operation and the channel occupation operation in the unlicensed band as described above will be described below with reference to FIG. 2, which illustrates a downlink transmission process of transmitting, by the base station, data or control signal to the terminal as an example, and the process may also be applied to uplink transmission in which the terminal transmits a signal to the base station.
An LTE subframe 200 in FIG. 2 is a subframe having a length of 1 ms, and may be configured of a plurality of OFDM symbols. The base station and the terminal capable of performing communication using an unlicensed band may perform communication by occupying a corresponding channel during a set channel occupancy time 250 and 260. When the base station occupying the channel for the set channel occupancy time 250 needs to additionally occupy the channel, the base station may perform the channel sensing operation 220, and then may reoccupy and use the channel depending on a result of the channel sensing operation. The required channel sensing period (or length) may be defined between the base station and the terminal in advance, set through a higher layer signal transmitted by the base station for the terminal, or set to be different according to a transmission/reception result of data transmitted through the unlicensed band.
Further, at least one of variables applied to the channel sensing operation that is performed again as described above may be set to be different from those of the previous channel sensing operation.
The operation for sensing and occupying a channel may be set to be different depending on the unlicensed band or regional and national regulations. The operation for sensing and occupying a channel with respect to a load-based equipment, which is one of channel access methods in EN301 893, a rule for 5 GHz spectrum of Europe, will be described in more detail below.
When the base station needs to additionally use the channel after the maximum channel occupancy time 250, it is required to determine whether other devices occupy the channel during a minimum channel sensing period 220. The minimum channel sensing period 220 may be set with a, depending on the maximum channel occupancy period, maximum channel occupancy period of 13/32×q, (q=4, . . . , 32) and a minimum channel sensing period of length of extended clear channel assessment (ECCA) slot x rand (1, q).
Here, the length of the ECCA slot is a predefined or preset minimum unit (or length) of the channel sensing period. That is, when q is set to 32, the transmitting device may occupy the unlicensed band for up to 13 ms. A minimum channel sensing period, a random value from 1 to q (that is, 1 to 32), may be selected, and a total channel sensing period may be the length of the ECCA slot x of the selected random value. Therefore, when the maximum channel occupancy period is increased, the minimum channel sensing period can also be increased. The method for setting the maximum channel occupancy period and the minimum channel sensing period is merely an example, may be applied differently depending on the frequency spectrum, and the defined regional and national regulations, and may be changed depending on frequency regulation amendments still to be determined. Further, an additional operation (e.g., introduction of additional channel sensing period) in addition to the channel sensing operation according to the frequency regulation may also be included.
When other devices using the corresponding unlicensed band is not sensed by the base station in the channel sensing period 220, that is, when the channel is determined to be in an idle state, the base station may immediately occupy and use the channel. The determination on whether other devices occupy the channel in the channel sensing period 220 may be performed using a predefined or preset reference value. For example, when intensity of a signal received from other devices during the channel sensing period is greater than a predetermined reference value (e.g., −62 dBm), it may be determined that the channel is occupied by other devices. When the intensity of the received signal is smaller than a reference value, the channel may be determined to be in the idle state. The method for determining whether the channel is occupied may include various methods such as the foregoing method using the size of the reception signal, a method of detecting a signal defined in advance, or the like.
Since a general LTE operation is performed in a subframe unit, the signal may not be transmitted or received in the specific OFDM symbol immediately after performing the channel sensing operation (e.g., a signal transmission and reception operation is performed from a first OFDM symbol of the subframe). Therefore, the base station sensing the idle channel in the channel sensing period 220 in the subframe as described above may transmit a specific signal 230 for channel occupancy from the point in time when the channel sensing period 220 ends to immediately before first OFDM symbol transmission of a next subframe, i.e., during a period 230. In other words, the base station may transmit a second signal (e.g., primary synchronization signal (PSS)/secondary synchronization signal (SSS)/cell-specific reference signal (CRS), a newly defined signal, etc.) for channel occupancy with respect to corresponding unlicensed band, synchronization of the terminal, etc., before transmitting a first signal (e.g., general ePDCCH and PDSCH) transmitted in the subframe 210 or 240. The transmitted second signal may not be transmitted depending on the channel sensing period ending point in time. Further, when a corresponding channel occupancy starting point in time is set within the specific OFDM symbol, a third signal (a newly defined signal) is transmitted to a next OFDM symbol starting point in time, and the second signal or the first signal may be transmitted. In the present disclosure, the channel sensing operation period is described using an OFDM symbol unit, but the channel sensing operation period may be set regardless of the OFDM symbol of the LTE system.
Here, the PSS/SSS currently used in the LTE system may be reused as the second signal, or the second signal may be generated using at least one of the PSS and the SSS by using root sequence currently used in the licensed band and other sequence. Further, the second signal may be generated using other sequences except the PSS/SSS sequence required to generate a physical cell ID (PCID) of the base station in the unlicensed band to be used without being confused with the physical cell ID of the base station. Further, the second signal includes at least one of CRS and CSI-RS currently used in the LTE system, or ePDCCH, PDSCH, or a signal having modified form of ePDCCH and PDSCH may be used as the second signal.
Since the period 230 in which the second signal is transmitted is included in the channel occupancy time, frequency efficiency may be maximized by allowing minimum information to be transmitted through the second signal transmitted in the period 230.
The LTE system (LAA or LAA cell) using the unlicensed band as described above requires a new channel access (or LBT) scheme that is different from the existing method of using the licensed band, in order to satisfy regulations on the unlicensed band to be used and coexist with other systems (wireless-fidelity (WiFi)) using the unlicensed band.
Referring to FIG. 3, the channel access scheme for using the unlicensed band of the WiFi system is now described.
When a WiFi AP1 310 has data to be transmitted to station 1 (STA1) or a terminal 1 315, a channel sensing operation for a corresponding channel can be performed to occupy the channel. Generally the channel is sensed during distributed coordination function (DCF) interframe space (DIFS) time 330. Whether the channel is occupied by other devices may be determined by various methods, e.g., using intensity of the signal received during the time 330, a method of detecting a signal defined in advance, or the like. When it is determined that the channel is occupied by another device 320 during the channel sensing time 330, the AP1 310 selects a random variable 355, e.g., N in a set contention window (e.g., 1-16). Generally, such operation is called a backoff operation. Then, the AP1 310 senses the channel during a predefined time (e.g., 9 us), and when it is determined that the channel is in the idle state, the selected variable N 355 can be subtracted by 1. That is, it is updated as N=N−1. When it is determined that the channel is occupied by the another device 320 during the time 330, the variable N 355 is not subtracted but is instead frozen. STA2 325 receives data 340 transmitted by the AP2 320 and transmits ACK or NACK 347 with respect to the reception of the data 340 to the AP2 320 after short interframe space (SIFS) time 345. The STA2 325 may transmit the ACK/NACK 347 without performing a separate channel sensing operation. After the transmission of the ACK 347 of the STA2 325 ends, the AP1 310 may know that the channel is in the idle state. The AP1 310 senses the channel during a predetermined time (e.g., 9 us) defined or set in advance for the backoff operation when it is determined that the channel is in the idle state for the DIFS time 350, and when it is determined that the channel is in the idle state, the selected variable N 355 is subtracted again. That is, it is updated as N=N−1. When N=0, the AP1 310 may occupy the channel to transmit the data 360 to the STA 1 315. Then, the terminal receiving the data 360 may transmit the ACK or NACK with respect to the reception of the data to the AP1 310 after the SIFS time. The AP1 310 receiving the NACK from the STA1 315 may select the random variable N used in the next backoff operation in the increased contention window. That is, when it is assumed that the contention window used is [1, 16], and the data reception result of the STA1 315 is NACK, the contention window of the AP1 310 receiving the NACK may be increased to [1, 32]. If the AP1 310 receives ACK in the above case, the contention window may be set to an initial value (e.g., [1, 16]) or a preset contention window may be decreased or maintained.
However, with a WiFi system, communication is generally performed between one AP (or base station) and one STA (or terminal) at the same time. Further, as 347 and 370 in FIG. 3, the STA1 and STA2 (or terminals 315, 325) transmits its data reception state (e.g., ACK or NACK) to the AP (or base station) immediately after the reception of the data. The AP 310 or 320 performs a channel sensing operation for the next data transmission operation after receiving ACK or NACK from the terminal 315 or 325. However, in the LAA system, one base station may transmit data to a plurality of terminals at the same time. Further, one or more terminals receiving the data at the same point in time (e.g., time n) may transmit ACK or NACK to the base station at the same time (e.g., n+4 in a case of FDD). Therefore, the LAA base station may receive the ACK or NACK from one or more terminals at the same point in time, unlike the WiFi system. In addition, time difference between the ACK/NACK transmission point in time of the terminal and the data transmission time of the base station may be at least 4 ms. Therefore, of the LAA base station sets (or resets) a contention window by the ACK/NACK received from the terminal like WiFi, since the base station may receive the ACK/NACK from a plurality of terminals at a specific time, uncertainty in setting the contention window may occur. Further, if the terminal performs an uplink channel sensing operation for uplink transmission, each terminal may independently perform the channel sensing operation. When the terminal independently performs the channel sensing operation as described above, only a terminal of which the channel sensing operation ends first may perform the set uplink transmission.
Accordingly, the present disclosure provides a method in which the base station sets a channel sensing period based on the uplink signal reception result received from the terminal, and performs a setting of the set channel sensing period for terminals such that the plurality of terminals may perform the channel sensing operation at the same time.